During a health maintenance visit, a 54-year-old man reports a four-month history of worse… — USMLE Step 2 CK

During a health maintenance visit, a 54-year-old man reports a four-month history of worsening sluggishness, generalized muscle aches, and dry skin. He mentions needing to wear a sweater even when indoors. His temperature is $36.0^\circ\text{C}$ ( $96.8^\circ\text{F}$ ), blood pressure is $112/74\text{ mmHg}$ , and pulse is $54/\text{min}$ . Physical examination reveals a puffy face, nonpitting edema of the lower extremities, and a notably delayed relaxation phase of the deep tendon reflexes. The thyroid gland is not palpable.

Laboratory evaluation shows the following:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$0.2\text{ }\mu\text{U/mL}$	$0.4-4.0\text{ }\mu\text{U/mL}$
Free thyroxine (T4)	$0.4\text{ ng/dL}$	$0.9-1.7\text{ ng/dL}$

Which of the following mechanisms best explains this patient's clinical presentation and laboratory findings?

Insufficient pituitary thyrotropin secretionCevap
B
Antibody-mediated destruction of thyroid follicles
C
Decreased hepatic synthesis of thyroxine-binding globulin
D
Peripheral tissue resistance to thyroid hormone
E
Unregulated ectopic production of thyroid hormone

Cevap

Insufficient pituitary thyrotropin secretion

The patient exhibits classic signs and symptoms of hypothyroidism, including bradycardia, delayed deep tendon reflexes, cold intolerance, and nonpitting edema (myxedema). The laboratory findings confirm hypothyroidism with a low free T4. However, instead of the expected compensatory rise in TSH seen in primary thyroid failure, the TSH is inappropriately low. This combination of low free T4 and low TSH defines central (secondary or tertiary) hypothyroidism, which is caused by insufficient secretion of thyrotropin (TSH) from the pituitary gland or thyrotropin-releasing hormone (TRH) from the hypothalamus.

Adım Adım Çözüm

Identify the clinical syndrome based on the patient's symptoms and physical exam.

The patient has fatigue, cold intolerance, dry skin, bradycardia, and delayed deep tendon reflexes, which are classic signs of clinical hypothyroidism.

Recognizing the clinical state guides the interpretation of the subsequent laboratory values.

Evaluate the laboratory results to determine the primary site of dysfunction.

The free T4 is clearly below the normal range, confirming hypothyroidism. The TSH is simultaneously low (

0.2\text{ }\mu\text{U/mL}

In primary hypothyroidism, the pituitary responds to low T4 by increasing TSH secretion. A low or inappropriately normal TSH in the setting of low T4 indicates central (secondary or tertiary) hypothyroidism.

Match the pathophysiologic mechanism to the central etiology.

Central hypothyroidism is caused by insufficient thyrotropin (TSH) secretion from the anterior pituitary (or insufficient TRH from the hypothalamus).

This confirms that the primary defect is a lack of stimulation to the thyroid gland, not a primary defect within the thyroid gland itself.

Anahtar Kavram

Central hypothyroidism is characterized by clinical signs of hypothyroidism accompanied by a low free T4 and a low or inappropriately normal TSH level, distinguishing it from primary hypothyroidism where TSH is elevated.

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